Generally speaking, photovoltaic systems are implemented to convert light energy into electricity for a variety of applications. Power production by photovoltaic systems may offer a number of advantages over conventional systems. These advantages may include, but are not limited to, low operating costs, high reliability, modularity, low construction costs, and environmental benefits. As can be appreciated, photovoltaic systems are commonly known as “solar cells,” so named for their ability to produce electricity from sunlight.
Conventional solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. Accordingly, conventional semiconductor layers generally absorb incoming light to produce excited electrons. In addition to the semiconductor layers, solar cells generally include a cover or other encapsulant, seals on the edges of the solar cell, a front contact electrode to allow the electrons to enter a circuit, and a back contact electrode to allow the ions created by the excitation of the electrons to complete the circuit.
One particular type of solar cell is a dye-sensitized solar cell. A dye-sensitized solar cell generally uses an organic dye to absorb incoming light to produce excited electrons. The dye sensitized solar cell generally includes two planar conducting electrodes arranged in a sandwich configuration. A dye-coated semiconductor film separates the two electrodes which may comprise glass coated with a transparent conducting oxide (TCO) film, for example. The semiconductor layer is porous and has a high surface area thereby allowing sufficient dye for efficient light absorption to be attached as a molecular monolayer on its surface. The remaining intervening space between the electrodes and the pores in the semiconductor film (which acts as a sponge) is filled with an organic electrolyte solution containing an oxidation/reduction couple such as triiodide/iodide, for example.
One exemplary technique for fabricating a dye-sensitized solar cell is to coat a conductive glass plate with a semiconductor film such as titanium oxide (TiO2) or zinc oxide (ZnO), for example. The semiconductor film is saturated with a dye and a single layer of dye molecules self-assembles on each of the particles in the semiconductor film, thereby “sensitizing” the film. A liquid electrolyte solution containing triiodide/iodide is introduced into the semiconductor film. The electrolyte fills the pores and openings left in the dye-sensitized semiconductor film. To complete the solar cell, a second planar electrode with low overpotential for triiodide reduction is implemented to provide a cell structure having a dye-sensitized semiconductor and electrolyte composite sandwiched between two counter-electrodes.
Conventional dye sensitized solar cells may be fabricated using planar layered structures, as set forth above. The absorption of light by the dye excites electrons in the dye which are injected into the semiconductor film, leaving behind an oxidized dye cation. The excited electrons travel through the semiconductor film by a “random walk” through the adjacent crystals of the film towards an electrode. During the random walk of the electron to the electrode, the electron may travel a significant distance, and the electron may be lost by combining with a component of the electrolyte solution, also known as “recombination.” Under irradiation by sunlight, the density of electrons in the semiconductor may be high such that such electron losses significantly reduce the maximum voltage and therefore the efficiency achievable by the solar cells. It may be advantageous to reduce the likelihood of recombination by reducing the travel path of the electron through the semiconductor and thereby reducing the length of time it takes for the electron to diffuse through the semiconductor to the conductive oxide of the electrode. One technique for reducing the travel distance of the electron is to reduce the thickness of the semiconductor film and thus, the distance the electron has to travel to reach an electrode. Disadvantageously, reduction in the thickness of the semiconductor film may reduce the light absorption in the dye, thereby reducing the efficiency of the solar cell.
Also, the injection of the electron from the dye into the semiconductor material leaves behind an oxidized dye cation. The oxidized dye is reduced by transfer of an electron from an iodide ion, leading to the production of triiodide that diffuse through the electrolyte solution to the back electrode where a catalyst supplies the missing electron thereby closing the circuit. The back electrode is generally carbonized or platinized to catalyze the electron transfer to the triiodide. The electrolyte solution is typically made in an organic solvent. Generally speaking, less volatile solvents, including ionic liquids, with a high boiling point are more viscous and impede the diffusion of ions to the point where the diffusion limits the power output and hence the efficiency of the solar cell. Such solvents may be advantageous in providing cell longevity, especially for cells fabricated on a polymer substrate, because polymer substrates may allow less viscous solvents having a low boiling point to diffuse out of the solar cell over time. Because the triiodide ion may originate from anywhere in the part of the electrolyte solution in contact with the dyed surface of the semiconductor, the ion may have to travel a long torturous path through the labyrinth created by the random pore structure of the semiconductor from near the front electrode to the back electrode to complete the circuit. These long paths may limit the diffusion current in the solar cell. Decreasing the travel distance of the ions may advantageously reduce the limitations caused by the slow diffusion of the ions. However, as previously described, reducing the thickness of the semiconductor film to reduce the ion transport path may disadvantageously reduce the light absorption of the dye.
Thus, while it may be advantageous to increase the thickness of the semiconductor film and thereby the surface area of the film to provide increased light absorption, the thicker the semiconductor film, the greater the distance the electrons and ions may have to travel to reach a respective electrode. Although longer light paths may be desirable to facilitate greater light absorption, the losses due to the increased recombination of the electrons into the semiconductor layer, as well as limits to current caused by slow ion diffusion through the electrolyte in the semiconductor pores, make the increased thickness of the semiconductor film disadvantageous since it may produce a less efficient solar cell.